During SCM checkout, the p4-plugin creates a p4 connection on both the master and the slave.

The user specified credential should not be expected to work on both the master and the slave - only on the node where the associated job is intended to run.

One example is a Jenkins master running on Linux and a Jenkins slave running on Windows. The job is configured to use a file based p4 tickets credential hosted on a windows network share. The Jenkins master would be unable to access that network share when specificed as \\path\to\network\share\p4tickets.txt.